There are many factors that affect the performance of silicon chips (CPUs, ASICs). Operating voltage affects the speed of transistors, with higher voltage switching faster but consuming more energy and lower voltage switching slower but consuming less energy. Random silicon variation from die to die affects the speed of silicon, power consumption, and minimum operating voltage. Temperature affects the switching speed of transistors as well. At typical operating voltages on modern silicon fabrication technologies, hot transistors switch faster but leak more energy while cool transistors switch slower but leak less energy.
As a result, for each silicon chip there are optimal operating points for temperature, voltage, and clock frequency (speed) that maximize either performance or performance per watt. Understanding and operating at the ideal point for each silicon chip results in higher return on investment (ROI) over the life of the chip.